For residents of South Hampton, New Hampshire, the first day of spring came not with an end to the deep snow and frigid temperatures but instead with unexpected consequences of a well-intentioned deed.

Locals had been feeding deer in the area in an effort to help get the animals through the harsh winter. But instead of sustaining them, the rapid change in diet from winter shrubs to carbohydrate-laced human food proved too much for the animals’ sensitive stomachs, and 12 deer died.

That’s just an example of what can happen when the wrong food is handed out to wildlife—a practice long shunned by conservationists.

In a new study published in the journal Ecology Letters, University of Georgia ecologists combined the findings of 20 studies on the impacts of feeding wildlife to see how, where, and why the practice was benefiting and harming animals.

As people move into undeveloped areas, habitat and natural food sources for animals decline, and interactions between humans and animals increase. That’s leading to more bird feeders in backyards and more elk feeding stations installed by state and national park managers to supplement winter diets. More animals are also sustaining themselves by way of our rubbish and landfills.

Each instance comes with its own set of complications.

“We wanted to know if there was an overall net tendency,” said Daniel Becker, a doctoral student at Georgia’s Odum School of Ecology and the study’s lead author. “We wanted to know what could explain the different responses.”

The results were a mixed bag.

Because bird feeders bring multiple species to one spot—not to mention the squirrels and other rodents that get in on the feast—the nutritious meal can be a hot spot for disease transmission.

“For intentional feeding sources like bird feeders, we expect parasites like bacteria and viruses to increase, so spacing these resources apart can help reduce the high contact rates driving transmission,” Becker said. “Cleaning feeders periodically can help limit the buildup of infectious stages in the environment that occurs when lots of animals become more sedentary.”

But in places where you would expect to find disease running rampant—landfills and garbage dumps—researchers found the opposite. Foxes feasting on garbage piles actually ingested fewer disease-spreading parasites, such as tapeworms and flukes, because they were eating fewer rodents, which are typically infected with worms.

But in the long run, making sure animals are eating the right, natural foods is most likely to boost their immune systems. Feeding them garbage can impede their health too.

One study looked at how tourists feeding grapes to iguanas in the Bahamas ended up altering their diet, leaving them susceptible to higher levels of infection to hookworm.

Still, Becker believes supplemental feeding of wildlife can be beneficial if the food source is healthy enough and is distributed properly. Those additional food sources could also be used to distribute vaccines or treatments into wild animal populations, further boosting immune systems.

“We need field experiments, we need long-term observational studies, and we need to develop models focusing on environmentally transmitted parasites like worms; those are areas where we’re lacking information,” he said. “This is an issue that’s not going away, so we need to understand it.”

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TakePart is the digital news and lifestyle magazine from Participant Media, the company behind such acclaimed documentaries as CITIZENFOUR, An Inconvenient Truth and Food, Inc. and feature films including Lincoln and Spotlight.